igc: Fix some issues in igc_neweitr()

The justification is the same as in commit
fb876eef219e ("e1000: Fix some issues in em_newitr()").

Reviewed by: kbowling
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D50548

(cherry picked from commit ef062029ceffacb6bde3a5639a2bd8c4d59ca1df)